Been fly fishing for Gill's on a 5wt (want to save it for trout) but what to get a 3wt. Im still a beginner to fly fishing so if I get a 3wt what size flies would i'll be able to cast and what size leader/tippet be suitable for a 3wt. 5x, 6x, 7x, 8x?

I use 3X & 5X tippet most of the time.
3X for size 10-12, and 5X for size 14-16.
I rarely use anything smaller than a #16 or larger than a #10 for 'gills.

I do make it a habit when tying to avoid getting my flies too heavy, though. I prefer wet flies that sink slowly instead of plummeting to the bottom. For example, I'll use bead chain eyes instead of lead eyes most of the time. Casting #10 flies is probably pushing the limit for the 3wt.

It depends on how far you want/need to cast. Bigger flies can be cast shorter ranges. I typically fish flies from size 8 and smaller with such rods. Bushy, wind resistant flies require more effort than sparser, streamlined patterns.

You can fish a #10 cork, deer hair, or foam popper very easily with a 3 weight.

#10 bushy dry flies are no problem for me either.

A big factor in fishing these flies is the right leader and tippet combo. You don't need anything more than a 7 foot leader for panfish with a 3 weight.

I typically fish 4X tippet for most fishing, 5X when I go to dry flies or small wets/nymphs from size 14 and smaller. I rarely have to step down to 6X unless I am trout fishing with very, very small flies.

I don't normally bump up to 3X with a light rod either - heck, I rarely fish 3X tippet on my 6 weight - only when fishing heavier flies in the size 4-8 range, depending on the fly. A 4X tippet for a #8 woolly bugger is good for me.

Are there any specific rods you're looking at? How much are you looking to spend?

The TQR rods are nice for a rod around a hundred bucks. If you wanted to save even more cash - the Three Forks 7'6" 3 weight is a good inexpensive rod - perfectly suited to bluegill and panfish. The rod itself is half the price of the TQR, with a similar action (the TQR is lighter and feels a bit smoother than current production Three Forks rods though)

You might also look at the Wright & McGill S-Curve Gen 2 rods - I've got a 7' 2 weight WMG that I'm really pleased with so far. It handles #2 or 3 lines very well, more of a medium-slow action rod than the faster action of the TQR rods. I haven't hooked up with a fish with it yet, but fished with it and it's such a sweet caster. It'll make a great bluegill rod for small poppers and dry flies.

I have a TFO signature 2wt 6' that I use for 90% of my fishing. I cast small popper, woolybuggers, many beadchain eye flies and medium to small streamers for SMB. I hope to find some carp this summer too. I can cast it all day. I am using a 4wt DT line and it almost casts itself. A pure pleasure to use and very versitile IMO. I also have cast #6 carp flies with ease. What more could you ask for in a light rod.

On my three weight I use flies from size 1 (rarely) down to size 24 (rarely), and most of my three weight fishing is using flies that range from 10-18. I do use small poppers and weighted flies such as bead heads and these present small problems with the extra weight, or wind resistance but nothing that can't be overcome easily.